This invention relates to a strain sensing device and more specifically to optically-active polymer clad fiber optics embedded in an organic matrix or organic matrix composite such as an epoxy composite. The strain sensing polymer-containing device consist essentially of diacetylene-containing polymers such as the diacetylene polyamides in combination with fiber optics capable of changing absorption in the visible spectrum while under strain. It is beneficial to have means for nondestructively evaluating strain resulting from the deformation of composites, e.g., fiber/epoxy laminates, in various structures and particularly in aerospace structures. Early warnings of excessive strain that might cause failure in flexible structures have many advantages.
There is now a growing interest in the area of sensor technology regarding the use of optical fibers as sensors to detect strain, pressure, temperature, etc.; see the Journal of Quantum Electronics, Volume QE-18, 1986. One means of measuring strain in composites using embedded optical fibers is disclosed by Claus et al., SPIE International Symposium, Volume 566, 1985. Here the work demonstrated the feasibility of an optical fiber interferometric strain measurement wherein the optical fibers were embedded in a composite laminate before the laminate was fabricated. The strain measurements were conducted to demonstrate that the system was functional and that there was reasonable correlation between the strain measured by the system and theoretical predictions. Further, the increased demand for strong flexible and light weight materials for the fabrication of various aircraft parts has driven the development of polymer based composites. These composites are useful as components of large structures, particularly aerospace structures. Thus, a nondestructive evaluation method is needed to determine the component's integrity not only during the manufacturing process but ultimately in the end use of the components. Moreover, because of the tremendous size of aerospace components, the nondestructive evaluation technique must be capable of characterizing a large surface area of the component.
In this regard, the prior art has considered embedding optical and acoustical wave guides as sensors for such large scale components. The acoustic wave guides have cross-sectional dimensions which are larger but still comparable with graphite fibers in an epoxy composite. Here the fibers may be embedded in the composite during the manufacturing process without changing the structure of the composite. These sensors provide there own mechanism for signal transfer and due to the potential dielectric nature of acoustic and optical wave link, the dielectric composition of the composite can be maintained. However, it should be noted that the diacetylene coated fiber sensors in accordance with this invention exhibit additional advantages over the acoustic sensors, particularly for large scale testing in that, for example, optical fiber attenuation for unit length is far less than that of acoustic rods.
In a graphite epoxy composite, e.g., the fiber bundle orientation from layer to layer alternates in order to give the material strength in several inplane directions. The spaces between the fibers in the bundles and between the bundles and between the layers in the composite are completly filled with the epoxy resin. The strain transfer from the material to the fiber depends also upon the mechanical properties of the fiber jacket. Although there has been much work done to identify jacketing materials having elastic constants to enhance the fiber pressure sensitivity, there has been little work done to determine the trade off between such enhancement and the effect of the jacket on the mechanical properties of the composite. The prior art also has embedded both bare optical fibers in a single pass straight length and polymer coated fibers in back and forth serpentine patterns between adjacent parallel, perpendicular and oriented composites; see the Journal of Nondestructive Evaluation 41,106 (1983) by R. O. Claus et al. Here the experiments used optical time domain reflectometry (OTDR) which required extensive electronic equipment to launch an optical pulse and to detect the pulse in the fiber. The amplitude of a pulse before and after the deformation are compared and then related to the strain. Other studies relied on the phase change as in relationship to the strain through a Mach-Zehnder Interferometric Measurement; see Journal of Composite Technology and Research 10,1 (1988).